1. Field of the Invention
The present invention broadly relates to the measurement of distortion in a transparency and, more particularly, is concerned with a system for measuring absolute angular deviation in transparencies, such as an aircraft windscreen or a helmet visor.
2. Description of the Prior Art
As a general rule, optically transparent, asymmetrically contoured bodies have been difficult to quantitatively evaluate and compare on the basis of their optical characteristics. A prime example of a structural element formed of a transparent medium in which optical quality is critical, yet difficult to quantitatively evaluate, is the canopy or windscreen of aircraft having complex curvilinear contours.
Distortion is one of the optical quality parameters that has been identified for characterizing transparencies, such as aircraft windscreens. Distortion is the non-linear mapping of object points to image space due to the optical effects of the transparency. Such effects may be due to either optical index variations in the transparency or to the opposite faces of the transparency being non-parallel. In the case of a perfect non-distorting transparency, a square grid object would be reproduced as an identical square grid in the image plane. Where there is index variation or non-parallelism between opposite faces of the transparency, the square grid will be reproduced in a distorted form in the image plane.
The visual effects of distortion, when present, are thus apparent even to untrained observers. However, quantifying the distortion to ascertain its severity is a difficult problem. Since distortion is the non-linear mapping of objects viewed through the transparency, the actual physical positions of the objects do not linearly correspond to their apparent locations as seen through the transparency. This effect can be especially detrimental to accurate aligning through an aircraft transparency of any device within the aircraft, for instance a heads-up display gun sight reticle with a target outside the aircraft.
In a more technical vein, distortion has been defined as the rate of change of angular deviation across the transparency. Angular deviation is defined as the angular deflection or change of direction of a light ray as it passes through the transparency. Theoretically, the distortion in any transparency may be determined by mapping at a plurality of locations on the transparency the angular deviation of light rays as they are transmitted from the object through the transparency to the observer.
Angular deflection of the light ray should not be confused with displacement of the light ray which is simply a lateral shift. Whenever a ray of light passes through a transparency at an angle other than normal (or perpendicular to the faces of the transparency), lateral displacement of the ray by a relatively small and constant amount results. However, whenever a ray of light passes through the transparency at the same angle, but where the opposite faces of the transparency are non-parallel, both lateral displacement and angular deflection result (FIG. 1). While lateral displacement is usually operationally insignificant beyond a few meters, the distance between the real location of the object and its apparent (deflected) position increases as the range of the object from the observer increases. Consequently, a method of quantifying the distortion in a transparency to ascertain its severity, especially in aircraft transparencies, is a necessity.
Several methods of measuring angular deviation have been employed in the past. One method uses a telescope or theodolite to view a reference target without the transparency. Then the transparency is interposed, and the target is viewed through several predetermined locations of the transparency. The difference in angular position of the target as seen with and without the transparency is a measure of the total deviation induced by the transparency. This total deviation measure unfortunately includes the effects of both lateral displacement and angular deviation. Also, measurements through areas of the transparency which cause image blurring or doubling are extremely difficult when using a telescope to look through such transparency areas.
An alternative to the telescope method involves projecting a laser beam through the transparency onto a screen. The position of the laser spot is compared to its position prior to insertion of the transparency, and the angular deviation may be calculated using trigonometry and the distance from the windscreen to the measurement screen. Although image blurring or doubling will influence the size and shape of the laser spot, it is easier to estimate the center of the distorted spot on the screen than to see the degraded image in the telescope-based system.
Both telescope-based and laser-based systems commonly reduce the contaminating effect of lateral displacement on angular deviation measures by viewing (or projecting) the image over a long distance--approaching 100 feet. For these systems, therefore, the total deviation measured is primarily due to the angular deviation and not the displacement. However, an obvious disadvantage of systems incorporating long "throw" distances in their test setups is the requirement for a large empty space, clear of obstructions, which may conflict with other functions of the facility in which the test apparatus is housed.
Furthermore, while more accurate angular deviation measurements may be obtained with a laser-based system such as the one disclosed in U.S. Pat. No. 4,249,823 to Harry L. Task and assigned to the assignee of the subject invention, a system of this particular type suffers from problems caused by the coherent nature of the laser beam in being projected upon a transmission diffraction grating. Specifically, the fan line of luminous energy generated by the grating has a speckled nature. This causes the reading produced by a linear detector array which detects the point at which the fan line crosses the array to incorporate random error within it.